1. Field of the Invention
The present invention relates to two dimensional mapping of fingerprint patterns for identification purposes utilizing a capacitive circuit array.
2. Description of the Prior Art
Fingerprint sensing and the associated identification systems which include data bases and a match algorithm processor have been available in the prior art. In the area of the fingerprint sensor, the primary prior art systems utilize an optical scanning method which is relatively bulky and expensive due to the optics, lasers and the CCD array utilized. Due to this reason, there have been attempts to develop electronic means of sensing fingerprint patterns. All electronic fingerprint sensors can be categorized as follows; tactile pressure sensors, thermal sensors and capacitive sensors. The first two categories are complex and expensive to make, and thus most of recent development activities are in the area of capacitive sensors.
One group of capacitive sensors rely on thin deformable membranes with metal electrodes (one electrode per pixel) coated underneath each membrane. If the membrane is very thin and can follow the finger surface deformation, the distance of the metal electrodes can be measured through capacitance means. However, such thin membranes are not durable, and hence, are not yet marketable.
Another group of capacitive fingerprint sensors read the capacitance difference from the rigid sensor electrodes to the finger surface ridges directly without relying on membranes. In this case, the capacitance difference due to the finger surface variation is minute, with a typical order of a few fF (femto farad), and is imbedded in the larger background and parasitic capacitance from the sensor structure and the readout lines. Therefore, somewhat complex circuitry such as those disclosed by Tartagni et al. in an article entitled xe2x80x9cFingerprint Sensor Based on the Feedback Capacitive Sensing Schemexe2x80x9d, IEEE Journal of Solid State Circuits, Vol. 33, p. 133 (January 1998) and U.S. Pat. No. 5,835,141 to Ackland et al. have been implemented to filter out the background capacitance. The circuitry utilized in Tartagni et al and Ackland et al involve several transistors, an amplifier or charge accumulation and transfers in every pixel, making the sensor array complex. The prior art all have one or two metal electrodes per pixel at the top most layer of the sensor.
What is thus desired is to provide a fingerprint sensor which maps fingerprints for identification purposes which is reliable, compact and is less expensive than existing fingerprint identification systems.
The present invention utilizes sensor electrodes, having three or more metal electrodes/lines per pixel at the topmost layer of the sensor; at least two input voltage lines of different phases and one or more of sensor pickup electrodes. This combination nulls out the DC background capacitance and thus no electronic circuitry below the sensor electrodes is required. The sensor array can be manufactured without the need for expensive and slow vacuum systems such as CVD""s (chemical vapor deposition) and diffusion machines, etc. In particular, the sensor array can be manufactured entirely on a low cost substrate, such as circuit boards, glass or ceramics, with thick/thin film processing methods requiring only 15-20 micron feature sizes. The array readout is achieved externally without requiring transistor switches or CCD""s in the pixel itself making the sensor device very economical to manufacture.